The laws of conservation of kinetic energy and momentum explain why a hard bouncing ball hitting a line of similar balls stops completely along with the other balls except the one at the far end, which moves away after the collision with substantially all the kinetic energy.
Similarly, the laws of conservation of kinetic energy and angular momentum explain the following: Suppose there is a bar of length 2R, with its center rotatably secured to a shaft of fixed position. Immediately in front of one end of the bar lies a ball of mass M. Now someone throws another ball of the same kind at the other end of the bar. If at the instant of collision, the inertia of the bar and each ball are the same relative to the shaft, i.e., if MR.sup.2 equals the inertia of the bar, then the incoming ball stops completely with the bar, and only the other ball is returned. If the bar is of a certain shape so that the other ball bounces back at a 45.degree. C. elevation, then it can travel high and far despite energy losses during the collision.